1. Field of the Invention
The present invention concerns a sensor for venous networks of a part of the living body, as well as a method for controlling such a sensor.
2. Description of Related Art
A sensor for venous networks of a part of a living body such as that of a finger, a hand, a toe, etc. is very much used, for example for controlling access to a site or a machine.
A venous network sensor comprises an infrared illumination source that illuminates a means for acquiring images from the sensor, such as for example a CCD camera.
This infrared illumination source is implemented by a set of infrared emitters that are normally positioned with respect to one another along rows and/or columns. Arrays or matrices of infrared emitters are then spoken of.
When a living body is placed between the infrared illumination source and the image acquisition means, some rays in the flow of light strike the image acquisition means directly and others strike this image acquisition means after having passed through the living body. The image thus acquired is then processed digitally so that the venous network of the living body appears on this image. A user can then be authenticated by comparing the venous network thus acquired and the venous network that he previously recorded.
Though the principle of a venous network sensor is simple, the use thereof may pose the problem of overexposure of the acquired image. This is because the rays that strike the image acquisition means directly cause an intensity of the pixels of the image that is great compared with that caused by the rays that pass through the living body. This difference in intensity causes artefacts on the acquired image, such as a light halo or local overintensities, especially at the edges of the living body. This is particularly true for systems capturing on the fly since the position of the object to be acquired may be constrained. These artefacts interfere with the processing operations that are applied to the acquired image in order to reveal the venous network.
To improve the quality of acquisition of the venous network, various solutions have been proposed. For example, a venous network sensor is known in which the activation of the infrared emitters is linked to the detection of the presence of a living body.
Such a venous network sensor does not give entire satisfaction.
The documents US-A-2005/047632, US-A-2008/06243 and US-A-2010/092047 disclose venous network sensors.